Castings for turbine blades are one example of an article which requires a surface finish. Such articles are commonly finished in vibratory environments.
A vibratory environment for providing a finish generally consists of a casing or tub, a volume of media which is sufficiently large to reduce the possibility of contact between articles disposed in the media and the articles themselves disposed in the media. The tub or casing is vibrated, causing the media to vibrate and circulate with respect to the article. The media exerts a frictional force on an area of the article which when coupled with relative movement between the article and media does work on the articles disposed in the media. For casings, the casing extends longitudinally. Media and articles are fed in at one end and are flowed through the casing much as water and small stone flow through a pipe. This causes the height of the media to vary with length and provides uncertainty as to the finish which is provided.
Processing articles in a tub or casing with other articles has another significant disadvantage. It is possible for one article to strike an adjacent article even though the statistical probability of such an event is small. In the case of turbine blade castings (hereafter referred to as turbine blades), such contact is particularly troublesome because of the high standards of quality which the blades must meet. A small scratch or nick in a turbine blade, which might be acceptable in another type of article, may cause scrapping of a casting which cost many hundreds of dollars to fabricate because of the tight tolerances required for the cast article.
Processing articles in a tub has an added disadvantage. Typically the machine is shut down after a sufficient processing time has elapsed to provide an adequate surface finish to the articles. This is followed by removing the articles from the tub and transporting the articles to the next location, all of which may inadvertently cause contact between turbine blades and result in nicks and scratches which require discarding the turbine blade.
Accordingly, scientists and engineers working under the direction of Applicants' assignee have sought to develop a vibratory finishing process which would eliminate or decrease the possibility of blade to blade contact and decreases the amount of handling the turbine blade casting experiences after the blade is finished while it is being moved to the next location.